Field of the Invention
The present invention relates to an optical system suitable for being used as an image pickup optical system for an image pickup apparatus such as a digital still camera, a video camera, a TV camera, and a monitoring camera.
Description of the Related Art
In recent years, an image pickup optical system which has a wide field angle and high performance (high resolution) over an entire screen has been required for an image pickup apparatus using a solid-state image pickup element. A retrofocus type image pickup optical system is known as the image pickup optical system having the wide field angle. Japanese Patent Application Laid-Open No. 2011-227124 discloses a retrofocus type wide angle lens with a single focal length, which includes a first lens unit having a negative refractive power, an aperture stop, and a second lens unit having a positive refractive power, and which has a photographing field angle of 120°.
In addition, a negative lead type zoom lens in which a lens unit having a negative refractive power leads other lens units (located closest to an object side) is known as the image pickup optical system with the wide field angle. Japanese Patent Application Laid-Open No. 2007-94174 discloses a zoom lens including a first lens unit having a negative refractive power, and a second lens unit having a positive refractive power. The zoom lens disclosed in Japanese Patent Application Laid-Open No. 2007-94174 is a zoom lens with a wide field angle in which an entire photographing field angle at a wide angle end is 114.7°, and a zoom ratio is about 1.65.
In recent years, the wide field angle and the high resolution have been strongly requested for the image pickup optical system which is to be used in the image pickup apparatus. For realizing the wide field angle and the high resolution in the negative lead type image pickup optical system described above, for example, it becomes important to appropriately set the refractive power and the lens structure of the first lens unit having the negative refractive power.
In general, in the negative lead type image pickup optical system, because the lens units constructing the negative lead type image pickup optical system are asymmetrically arranged, various aberrations such as field curvature and distortion are abundantly generated, and hence these aberrations become difficult to correct. In particular, for the image pickup optical system having the wide field angle region, the refractive power of the first lens unit having the negative refractive power on the object side needs to be strengthened, and hence the aberrations described above are abundantly generated. For this reason, for satisfactorily correcting the distortion aberration and the field curvature while the wide field angle is realized in the negative lead type image pickup optical system, to thereby obtain the high optical characteristic, it becomes important to appropriately set the lens structure of the first lens unit having the negative refractive power.
On the other hand, in the image pickup optical system with the wide field angle, the shapes of the first lens arranged closest to the object side, the second lens arranged adjacent to the image side of the first lens, and the like are set as meniscus shapes, and curvature radii of the lens surfaces of these lenses are reduced, to thereby realize the wide field angle. In particular, in the image pickup optical system with the wide field angle in which the photographing field angle is about 100°, the curvature radii of the lens surfaces of the first lens and the second lens become small in many cases because the negative refractive powers of the first lens and the second lens are strengthened.
In this case, of the light fluxes from the object, the light flux reflected by the lens surface on the object side of the second lens is reflected by the lens surface on the object side of the first lens or the lens surface on the image side of the first lens to reach an image plane in some cases. Such a light flux causes a ghost and flare, which leads to reduction of the image quality of the photographed image. For this reason, in the image pickup optical system with the wide field angle, it is important to appropriately set the shape of the meniscus-shaped negative lens located closest to the object side.